Convert List<Person> to Map<Integer, List<Integer>> using Lambdas

chmk :

I wanted to convert list to Map as below. Here is the example.

I have student list something like below code snippet. Get a Hasmap out of it with Key as Integer which is Age and value as List.

From the below input feed, My response should be something like below. How do I achieve this ?

Map[[10, {1}], [20, {2,3,4}], [30,{5}]. [40,{6}]];

private static List<Person> getPersonTestData() {
    List<Person> personList = new ArrayList<>();
    personList.add(Person.of(1, "First1", "Last1", 10));
    personList.add(Person.of(2, "First2", "Last2", 20));
    personList.add(Person.of(3, "First3", "Last3", 20));
    personList.add(Person.of(4, "First4", "Last4", 20));
    personList.add(Person.of(5, "First5", "Last5", 30));
    personList.add(Person.of(6, "First6", "Last6", 40));

    return personList;
}

Thanks In Advance.......!

Naman :

You can do it using groupingBy and mapping in the downstream as :

Map<Integer, List<Integer>> ageToIdsMap = getPersonTestData().stream()
        .collect(Collectors.groupingBy(Person::getAge,
                Collectors.mapping(Person::getId, Collectors.toList())));